Method of fabricating MEMS devices (such as IMod) comprising using a gas phase etchant to remove a layer

ABSTRACT

Improvements in an interferometric modulator that cavity defined by two walls.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a divisional of application Ser. No. 10/752,140,filed on Jan. 5, 2004, now U.S. Pat. No. 7,692,844, which is adivisional of application Ser. No. 09/966,843, filed on Sep. 28, 2001,now Pat. No. 6,867,896, which is a divisional of application Ser. No.09/056,975, filed on Apr. 8, 1998, now Pat. No. 6,674,562. Thisapplication is also related to application Ser. No. 08/769,947, filed onDec. 19, 1996, now abandoned, application Ser. No. 08/554,630, filed onNov. 6, 1995, now abandoned, and application Ser. No. 08/238,750, filedon May 5, 1994, now Pat. No. 5,835,255. All of the above-referencedpatents and patent applications are hereby incorporated by reference intheir entireties.

BACKGROUND

This invention relates to interferometric modulation.

Interference modulators (IMods) are a broad class of devices thatmodulate incident light by the manipulation of admittance via themodification of the device's interferometric characteristics.Applications for such devices include displays, optical processing, andoptical information storage.

SUMMARY

In general, in one aspect, the invention features an interferometricmodulator comprising a cavity defined by two walls. At least two armsconnect the two walls to permit motion of the walls relative to eachother. The two arms are configured and attached to a first one of thewalls in a manner that enables mechanical stress in the first wall to berelieved by motion of the first wall essentially within the plane of thefirst wall.

Implementations of the invention may include one or more of thefollowing features. The motion of the first wall may be rotational. Eachof the arms has two ends, one of the ends attached to the first wall anda second end that is attached at a point that is fixed relative to asecond one of the walls. The point of attachment of the second end isoffset, with reference to an axis that is perpendicular to the firstwall, from the end that is attached to the second wall. The first wallhas two essentially straight edges and one end of each of the arms isattached at the middle of one of the edges or at the end of one of theedges. A third arm and a fourth arm also each connects the two walls.The arms define a pinwheel configuration. The lengths, thicknesses andpositions of connection to the first wall of the arms may be configuredto achieve a desired spring constant.

In general, in another aspect, the invention features an array ofinterferometric modulators. Each of the interferometric modulators has acavity defined by two walls and at least two arms connecting the twowalls to permit motion of the walls relative to each other. The wallsand arms of different ones of the modulators are configured to achievedifferent spring constants associated with motion of the walls relativeto each other.

In general, in another aspect, the invention features a method offabricating an interferometric modulator, in which two walls of a cavityare formed, connected by at least two arms. After the forming, a firstone of the walls is permitted to move in the plane of the first wallrelative to the arms to relieve mechanical stress in the first wall.

In general, in another aspect, the invention features an interferometricmodulator comprising three walls that are generally parallel to oneanother. The walls are supported for movement of at least one of thewalls relative to the other two. Control circuitry drives at least oneof the walls to discrete positions representing three discrete states ofoperation of the modulator.

Implementations of the invention may include one or more of thefollowing features. In one of the three discrete states, there is a gapbetween the first and a second of the two walls and a gap between thesecond and a third of the two walls. In a second of the three discretestates, there is a gap between the first and the second of the two wallsand no gap between the second and the third of the two walls. In thethird of the three discrete states, there is no gap between the firstand the second of the two walls and no gap between the second and thethird of the two walls. Each membrane includes a combination ofdielectric, metallic, or semiconducting films.

In general, in another aspect, an interference modulator includes acavity defined by two walls that are movable relative to one another toand from a contact position in which the two walls are essentiallyadjacent to one another. Spacers are mounted to form part of one of thewalls to reduce the surface area over which the two walls touch in thecontact position.

Implementations of the invention may include one or more of thefollowing features. The spacers comprise electrodes and conductors feedcurrent to the electrodes.

In general, in another aspect, the invention features an interferencemodulator comprising a cavity defined by two walls that are separated bya fluid-filled gap. The walls are movable relative to each other tochange the volume of the gap. An aperture (e.g., a round hole in thecenter) in one of the walls is configured to control the damping effectof fluid moving into or out of the gap as the volume of the gap changes.In implementations of the invention, the aperture comprises a round holein the center of the wall.

In general, in another aspect, the invention features an interferencemodulator comprising at least two walls that are movable relative toeach other to define a cavity between them. The relative positions ofthe walls define two modes, one in which the modulator reflects incidentlight and appears white and another in which the modulator absorbsincident light and appears black. In implementations, one of the wallsmay include a sandwich of a dielectric between metals, and the other ofthe walls may comprise a dielectric.

In general, in another aspect, the invention features an interferometricmodulator comprising a cavity defined by two walls with at least twoarms connecting the two walls to permit motion of the walls relative toeach other. The response time of the modulator is controlled to apredetermined value by a combination of at least two of: the lengths ofthe arms, the thickness of one of the walls, the thickness of the arms,the presence and dimensions of damping holes, and the ambient gaspressure in the vicinity of the modulator.

In general, in another aspect, the invention features an interferometricmodulator comprising a cavity defined by two walls, at least two armsconnecting the two walls to permit motion of the walls relative to each.The modulator includes a charge deposition mitigating device includes atleast one of actuation rails or the application of alternating polaritydrive voltages.

In general, in another aspect, the invention features an interferometricmodulator comprising a cavity defined by two walls held by a supportcomprising two materials such that the electrical or mechanicalproperties of the mechanical support differ at different locations in across-section of the mechanical support.

Implementations of the invention may include one or more of thefollowing features. The support may include a laminate of two or morediscrete materials or a gradient of two or more materials. The twomaterials exhibit respectively different and complementary electrical,mechanical, or optical properties.

In general, in another aspect, the invention features, a method for usein fabricating a microelectromechanical structure, comprising using agas phase etchant to remove a deposited sacrificial layer. Inimplementations of the invention, the MEMS may include an interferencemodulator in which a wall of the modulator is formed on the substrateand the gas phase etchant may remove the sacrificial layer from betweenthe wall and the substrate. The gas phase etchant may include one of thefollowing: XeF2, BrF3, ClF3, BrF5, or IF5.

In general, in another aspect, the invention features a method of makingarrays of MEMS (e.g., interference modulators) on a production line.Electronic features are formed on a surface of a glass or plasticsubstrate that is at least as large as 14″×16″, and electromechanicalstructures are micromachined on the substrate. In implementations of theinvention, the steps of forming the electronic features overlap (or donot overlap) with steps of micromachining the structures.

Other advantages and features will become apparent from the followingdescription and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a double clamped IMod.

FIG. 1B is a perspective view of an IMod with pinwheel tethers and adamping hole.

FIG. 1C is a top view of an IMod with pinwheel tethers and a dampinghole.

FIG. 1D is a top view of an IMod with straight tethers.

FIG. 2A shows a perspective view of a black and white IMod.

FIG. 2B shows a side view of the IMod in two states.

FIG. 2C illustrates the thin film structure of the IMod.

FIG. 2D shows the spectral reflectance function of the IMod in its twostates.

FIG. 3A shows a perspective view of a multi-state IMod.

FIG. 3B shows a top view.

FIG. 3C shows a side view of the IMod in three states.

FIG. 3D illustrates the thin film structure of the IMod.

FIGS. 3E, 3F, and 3G show spectral reflectance functions of agreen/white/black IMod, a red/white/black IMod, and a blue/white/blackIMod, respectively.

FIG. 4A shows the relationship between the multi-state IMod's states andthe drive voltage.

FIG. 4B shows the related electromechanical hysteresis curves.

FIG. 4C illustrates part of a drive circuit to drive or actuate a deviceaccording to the multiple states of FIG. 4A.

FIG. 5A shows an IMod, illustrating the effects of charge injection, inthe undriven state.

FIG. 5B shows the IMod driven.

FIG. 5C shows the IMod undriven after charge transfer.

FIG. 5D shows the IMod with reverse polarity applied.

FIG. 5E shows the IMod shows a reduced area electrode configuration,which reduces the effects of charge injection, as well as providing ahigher resistance to electrical shorts.

FIG. 6 is a side view of two IMods illustrating a mechanism to alter thespring constant.

FIG. 7A shows a single material membrane tether support.

FIG. 7B shows an alloyed or graded material membrane tether support.

FIG. 8 is a diagram of layers of a modulator.

FIG. 9 is a perspective view of cavities in a device.

FIG. 10 is a diagram of a side view of a pixel device.

FIG. 11 is a graph of the optical response for a cavity which appearsblack.

FIG. 12 is a graph of the optical response for a cavity which appearsblue.

FIG. 13 is a graph of the optical response for a cavity which appearsgreen.

FIG. 14 is a graph of the optical response for a cavity which appearsred.

FIG. 15 is a graph of the optical response for a cavity which appearswhite.

FIG. 16 is a perspective view of a fragment of a reflective flat paneldisplay.

FIGS. 17A, 17B, 17C, and 17D are perspective views of different spacersduring fabrication.

FIGS. 18A, 18B, 18C, and 18D are also perspective views of differentspacers during fabrication.

FIGS. 19A, 19B, 19C, 19D are top views of a static graphic image.

DESCRIPTION

The optical impedance, the reciprocal of admittance, of an IMod can beactively modified so that it can modulate light.

The modulation effect on incident radiation is described below. Thebinary modulation mode is shown in FIG. 9. In the undriven state (shownin back) incident light (which may include a range of incidentfrequencies, e.g., the range of visible light) contains a spectralcomponent which is at the resonant frequency of the device in theundriven state. Consequently this component is transmitted and theremaining components (at nonresonant frequencies) are reflected. Thisoperation is in the nature of the operation of a fabry-perotinterference cavity.

When the device is driven and the geometry altered to collapse (shown infront), the resonant frequency of the device also changes. With thecorrect cavity dimensions, all of the incident light is reflected.

A device may also be operated in an analog mode, where a continuouslyvariable voltage may be used to cause a continuously variable degree oftranslation of a secondary mirror/conductor. This provides a mechanismfor continuous frequency selection within an operational range becausethe resonant frequency of the cavity can be varied continuously.

$\begin{matrix}{T = {\frac{T_{a}T_{b}}{\left\lbrack {1 - \left( {R_{a}^{-}R_{b}^{+}} \right)^{1/2}} \right\rbrack^{2}}\mspace{14mu}\left\lbrack {1 + {\frac{4R_{a}^{-}R_{b}^{+}}{\left\lbrack {1 - \left( {R_{a}^{-}R_{b}^{+}} \right)^{1/2}} \right\rbrack^{2}}\mspace{14mu}\sin^{2}\mspace{14mu}\left( {\frac{\phi_{a} + \phi_{b}}{2} - \delta} \right)}} \right\rbrack}^{- 1}} & (1)\end{matrix}$δ=(2πnsds cos θs)/λs   (2)T=Tb for Ta approaching 0  (3)

The equations shown above explain the performance of the cavityaccording to one embodiment. Equation 1 defines the transmission Tthrough a fabry-perot cavity. Ta, Tb, Ra, Rb are the transmittances andreflectances of the primary (a) and secondary (b) mirrors. Phi a and Phib are the phase changes upon reflectance at the primary and secondarymirrors, respectively. Delta is the phase thickness. Equation 2 definesthe phase thickness in terms of the cavity spacing ds, the index ofrefraction of the spacer ns, and the angle of incidence, theta s.Equation 3 shows that the transmission T becomes the transmission of thesecond mirror when the transmission of the first mirror approaches 0.

One way of doing this (some aspects of which are described in U.S.patent application Ser. No. 08/238,750 filed May 5, 1994, andincorporated by reference) is by a deformable cavity whose opticalproperties can be altered by deformation, electrostatically orotherwise, of one or both of the cavity walls. The composition andthickness of these walls, which comprise layers of dielectric,semiconductor, or metallic films, allow for a variety of modulatordesigns exhibiting different optical responses to applied voltages. Thisscheme can be considered a form of microelectromechanicalstructure/system (MEMS).

Another way of actively modifying the impedance of an IMod (some aspectsof which are described in U.S. patent application Ser. No. 08/554,630,filed Nov. 6, 1995, and incorporated by reference) relies on an inducedabsorber to regulate the optical response. Such an IMod may operate inreflective mode and can be fabricated simply and on a variety ofsubstrates.

Both the deformable and induced absorber schemes typically work in abinary mode, residing in one of two states, or an analog or tunablemode, residing in one of a continuous range of states. The differencebetween these two modes is based primarily on the mechanical design ofthe IMod structure.

Some applications could use a multi-state IMod that can reside in morethan two states based on its mechanics and structure. A multi-state IModcan offer several advantages from both an optical performance anddigital driving perspective.

Structural components in MEMS may exhibit residual film stress, thetendency of a deposited film, say of aluminum, to either shrink andcrack (tensile stress) or push outward and buckle (compressive stress).A variety of factors contribute to the nature and magnitude of thisstress. They include parameters of the deposition process as well as thetemperature of the substrate during the deposition.

Control of this stress determines, in part, the forces required toactuate the structures as well as the final shapes of the structures.For example, a self-supporting membrane with very high residual stressmay require prohibitively high driving voltages to actuate. The samemembrane also may twist or warp due to these forces.

Actuation voltage, electromechanical behavior, and final shape areimportant characteristics of IMods. Some device applications exploit theelectromechanical properties. Large area displays, for example, can takeadvantage of the inherent hysteresis of these structures in order toprovide “memory” at the pixel location. However this requires that theIMods in a given array behave in a nearly identical fashion. Since theirbehavior is determined by the mechanical properties of the materials,among them residual stress, the films must be deposited with greatconsistency over the area of the display. This is not always readilyattainable.

FIG. 1A is an illustration of one IMod structural design, which has beendiscussed in previous patent applications. This design can be describedas a “double clamped” beam in that it consists of a self-supporting beamgo which is supported, or clamped, on both ends 92. When this structureis subject to residual stress, the height of the membrane (the beam) canincrease or decrease depending on whether the stress is compressive ortensile respectively. In FIG. 1A, membrane 90 is shown in a state oftensile stress, which causes the membrane to shrink in area. Because thestructure is bound to the substrate at points 92, the membrane height isdecreased due to this shrinkage. Conversely membrane 94, shown incompressive stress, attempts to expand with the end result being a netincrease or decrease in height or overall bowing of the structure.

FIG. 1B shows an improvement to this design. In this case, the movablesecondary mirror 100 is connected to support posts 104 via tethers 102.The IMod is fabricated on substrate 106, and incorporates stiction bumps108. The structure has advantages with respect to residual stress. Inparticular, because tethers 102 are tangential to secondary mirror 100,residual stress in the material will have a tendency to be relieved bycausing the mirror 100 to twist in a clockwise direction or counterclockwise direction if the stress is compressive or tensile.

This twist is illustrated for a tensile case in FIG. 1C. Because atensile film has a tendency to shrink, the sides of secondary mirror 100are pulled towards the support posts 104 with which they are associated,while the mirror remains in its original plane. The twisting relievesthe residual stress of the structure. This stress relief occurs afterthe last step of the IMod fabrication when a supporting sacrificialspacer is removed from beneath the structure. Depending on the overalldesign of the IMod, a certain amount of structural rotation can betolerated. Consequently, minute variations of residual stress across theexpanse of a display array are mitigated or eliminated because each IModrotates to its individual stress relieved position, all withoutaffecting the optical properties.

The other consequence of this relief is that stress no longercontributes, or contributes much less, to the electromechanical behaviorof the device. Device characteristics such as voltage and resonantfrequency are thus determined primarily by factors such as modulus ofelasticity and film thickness. Both of these characteristics are moreeasily controlled during deposition.

FIG. 1D illustrates another geometry for a stress relieving structurerelying on straight tethers 102. In this case, the mirror is rotatingclockwise to relieve compressive stress. Other tether configurations,including curved or folded, are also possible,

Referring again to FIG. 1B, a micro-electromechanical structure has atendency to stick to a surface of a substrate that it touches duringoperation. Structures that minimize the area of contact between movablemembrane 100 and the substrate can mitigate this phenomenon. Stictionbumps 108 can provide this mechanism by acting as supports which contactthe membrane only over a relatively small area. These structures can befabricated using the micromachining techniques described in the previouspatent applications. They can also act as bottom electrodes if suitablyinsulated, and exhibit certain advantages over previously describeddesigns, which will be discussed below. In this role they may bereferred to as actuation rails. These structures may also be fabricatedon the movable membrane.

Referring again to FIG. 1B, damping hole 110 also enhances theperformance of this structure. When the membrane is actuated i.e.,pulled downward, the air between it and the substrate must be displaced.The same volume of air must be replaced when the membrane is allowed todeflect back to its quiescent position. The energy required to move thisvolume of air has the effect of slowing the motion of the membrane ordamping its behavior. Damping is both a detriment and an advantage.Minimizing the response time of these devices is important in order tosupport the necessary display data rates, thus the desire exists tominimize damping. However it is also important to bring the membrane tofixed position very quickly in order to reduce the amount of lightreflected, over time, which is not of the desired color. Withinsufficient damping, such a membrane can experience ringing, ordecaying oscillation, when it is released into the undriven state. Thisshould be minimized, and is also determined in part by damping.

One method of optimizing damping is to provide a damping hole throughthe body of the membrane. The hole serves to provide a supplementarypath for the air during the motion of the membrane. The force requiredto displace and replace the air is thus lessened, and the effect ofdamping reduced. Thus choosing the size of the hole during manufactureprovides a mechanism for manipulating the amount of damping the IModexperiences, and therefore its response time. Stiction bumps, 108, canalso assist in minimizing damping. They do so by maintaining a finitedistance between the membrane and substrate so that there is a path forairflow, between the membrane and the substrate, when the membrane isfully actuated.

Another method for optimizing damping relies on control of the ambientgas pressure. Any IMod device, as described in previous patentapplications, will be packaged in a container that provides a hermeticseal, using an inert gas. This prevents the introduction of bothparticulate contaminants as well as water vapor, both of which candegrade the performance of the IMod over time. The pressure of this gashas a direct bearing on the amount of damping that the packaged deviceswill experience. Thus, the damping, and response time, may also beoptimized by determining the ambient gas pressure within the packagingduring manufacture.

A key metric of performance in a reflective flat panel display is itsbrightness. Most of these displays achieve color spatially, that is eachpixel is divided into three sub-pixels corresponding to the colors red,blue, and green. White is achieved by maximizing the brightness of allthree sub-pixels. Unfortunately, since each sub-pixel utilizes onlyabout ⅓ of the light incident upon it, the overall brightness of thewhite state can be low.

This can be resolved by utilizing a sub-pixel structure that is capableof directly achieving a white state, in addition to a particular color.In this fashion, the overall brightness of the display can be increasedbecause a sub-pixel in a white state utilizes a significantly higherfraction of the light incident upon it. The IMod design described inpatent application Ser. No. 08/554,630 is capable of reflecting either aparticular color or exhibiting a “black” or absorbing state. This designcan be modified to include alternative states.

FIG. 2A shows a perspective view of an arrangement that is capable of ablack state and a white state, and illustrates the previously describedtether configuration. (The double-clamped membrane of FIG. 1A is also ausable mechanical design though with the mentioned sensitivities tostress.) FIG. 2B shows the IMod in the two states with 204 being theundriven state, and 206 being the driven state. In the driven state theIMod absorbs incident light and appears black to a viewer lookingthrough substrate 202. In the undriven state, the IMod appears white.

FIG. 2C reveals details of the films involved. Movable membranes 208,210, and 212, comprise three films of a metal, a dielectric, and ametal, respectively. One example could utilize aluminum of 400nanometers (nm) thick for metal 208, silicon dioxide of 50 nm fordielectric 210, and tungsten of 14.9 nm for metal 212. Dielectric 214could comprise a film of zirconium dioxide 54.36 nm thick, residing onsubstrate 26. FIG. 2D illustrates the spectral reflectance function ofthis IMod design in the two states. Curves 216 and 218 reveal thereflectivity of the IMod in the white state and the black state,respectively

FIG. 3A is a variation that is capable of three states. In this design,the thin film stack of the design in FIG. 2A has been broken intoseparate movable membranes. Membrane 300 is a metal, 400 nm of aluminumin this case, and membrane 302 is also a metal, 14 nm of tungsten forexample. Because the tungsten is so thin, optically neutral structuralfilms may be required to provide the requisite mechanical integrity,which could be in the form of a supporting frame. The air gap betweenthe two membranes functions as the dielectric. FIG. 3B shows a top viewof this IMod revealing detail of how actuation would occur. Onecomplication of this design is that conducting membrane 302 shieldsmembrane 300 from the electric fields produced by the stiction/actuationbumps. Lengthening membrane 300 at regions 303, 304 so that it extendsbeyond the footprint of membrane 302 allows membrane 300 to “see” theelectric fields via paths 305, 307 and thus be acted upon by them.

The three possible mechanical states, and associated dimensions, areillustrated in FIG. 3C. Airgap dimensions 308 and 310 could be 215 nmand 135 nm. FIG. 3D reveals detail of the thin films involved. Film 320is a metal, 322 is an airgap which serves as a dielectric, 324 is also ametal, and 326 is a dielectric. FIG. 3E is a spectral reflectance plotof the three states. For the dimensions indicated, a black state (e.g.state 2), a blue state (state 0), and a white state (state 1) arepossible, with the black, blue and white states corresponding tospectral reflectance plots, 334, 332, and 330. FIG. 3F shows plots foran IMod with green and white states 336 and 334, while FIG. 3G showsplots for an IMod with red and white states 340 and 338.

Like all IMods, this design exhibits electromechanical hysteresis,though it is more complicated than an IMod with only two states. Thereis a minimum voltage which, when applied, is sufficient to keep one orboth membranes in a driven or actuated state despite the mechanicalforces which seek to return them to their relaxed positions.

FIG. 4A is a representative plot showing the relationship betweenapplied voltage and the state of the IMod. A minimum bias, Vbias, isrequired to maintain the IMod in the state into which it has beendriven. State 1 and State 2 are achieved by the application of voltagesV 3 and V 4. The related hysteresis diagram is shown in FIG. 4B, withcurve 400 corresponding to the electromechanical response of movableplate 302 of FIG. 3A, and curve 402 corresponding to that of movableplate 300. Vbias resides at the average of the centers of the twocurves. FIG. 4C illustrates one part of a drive circuit required toactuate such a device. Output stage 406 consists of three transistors orother suitable switches that are connected in parallel to threedifferent voltage sources, and the two movable plates of the IMod.Driver logic 404 responds to input signals 408 in a way that allows forthe selection, via the output stage, of one particular voltage to beapplied to the movable membranes of IMod 410. When no voltage isapplied, the IMod's membranes move to their relaxed state via mechanicalforces.

Another issue that can be encountered in movable membrane structures isthat of charge deposition, a phenomenon illustrated in FIGS. 5A-5C. InFIG. 5A, a voltage is applied between movable plate 500 and fixed plate504. Layer 502 is an insulating film that resides on top of fixed plate504. If the applied voltage is sufficient to actuate the movable plateand it comes into contact with the insulator, as it does in FIG. 5B, itmay deposit charge 506 on the insulator. One consequence of this is thatthe attractive force between plates 500 and 504 is reduced, and a highervoltage must be applied in order to achieve actuation (FIG. 5C).

This condition can be resolved by applying alternating voltages to thestructure. That is, for every intended actuation, change the polarity ofthe voltage that is applied such that the deposited charge is canceledout or actually exploited. FIG. 5D illustrates the effect of applying areverse polarity. The other alternative is to eliminate the solidinsulator and replace it with air. FIG. 5E illustrates the use ofstiction bumps or actuation rails to accomplish this goal. Charge maystill accumulate on these structures, but the area is much smaller, andtherefore the accumulated charge is decreased. Reverse polarity andstiction bumps may also be used together.

Electrical shorts are another concern for these devices. Referring againto FIG. 5A, the surface area of both the movable membrane (topelectrode) 500 and the bottom electrode 504 are equivalent. When thedevice is actuated (FIG. 5B), pinholes in the insulator, 502, could leadto electrical shorts and device failure. Utilizing a configuration likethat shown in FIG. 5E can mitigate this issue by reducing the surfacearea of the surface electrode so that the probability of a shortproducing pinhole is reduced. The surface electrode, orstiction/actuation rail, serves the aforementioned function of stictionmitigation as well. Like stiction bumps, they may be fabricated on themovable membrane instead.

Another issue that complicates the fabrication of a display based onIMods is the manufacturing of a full-color display. Since differentcolors in an IMod are achieved by the undriven spacing of the IMod, anarray with three different colors will have subarrays of IMods withthree different gap sizes. Consequently, there will be three differentelectromechanical responses for the driving electronics to contend with.The damping holes are one technique for compensating for the variationin electromechanical response from color to color.

Another technique is to vary the thickness of either the membrane, inthe double clamped IMod, or the tether thickness in the tether supportedIMod. The latter technique is illustrated in FIG. 6. Tether 600 on IMod602 is fabricated so that it is thinner than tether 604 on IMod 606.With the same bias voltage applied to both, IMod 602 is displacedfurther than IMod 606 because of its lower spring constant. Less forceis required to actuate this structure and its mechanical response timeis lower, and it is the mechanical response time that tends to dominate.This effectively changes the overall electromechanical response of thedevice and thus provides a way to compensate for spacing variation. Thesame technique applies to the double clamped design only the thicknessof the entire membrane, or major parts of it, are varied. By way ofexample, an IMod that is red and therefore has a longer mechanicalresponse time because of the greater undriven spacing, can be fabricatedwith a higher spring constant. This makes it possible to match itsactuation time to that of, say, the blue IMod.

In the tether supported IMod, the spring constant could be determined bylengths of the tether arms. A longer tether results in a lower springconstant and a shorter tether produces a higher constant. This could beaccomplished, in the same amount of total device space, by varying theposition along the edge of the movable membrane to which the tether isattached. Thus, a tether connected to the center of the membrane edgewould have a lower (a higher) than one connected to the nearer (thefarther) end, respectively.

The concept of decoupling the optical properties of the movable membranefrom the structural properties was discussed in the previous patentapplication. The fundamental idea is to fabricate a structure withseparate elements designed and optimized to provide the requiredmechanical and structural characteristics and, independently, therequired optical properties.

FIG. 7A reveals more detail about one possible approach. In this casethe movable membrane, 700, is selected purely on the basis of it opticalproperties and membrane tether, 702, for its advantageous mechanicalproperties. Aluminum, for example, has already been shown to be usefulin several IMod designs from an optical perspective, though mechanicallyit is subject to fatigue and stress fractures. A more suitable materialmight be a dielectric like aluminum oxide, silicon oxide or siliconnitride, which could be used to construct the tether.

FIG. 7B illustrates a variation on the theme where the tether iscomposed of either a laminated or graded material. In a laminatedmaterial, layers 706 and 710 might comprise films of aluminum oxide,providing good mechanical strength, and film 708 could be aluminum,providing electrical conductivity. For a graded material, layers 710-706could be composed of a continuously varied material that is deposited sothat at the inner surface it is pure aluminum, and at the outer surfaceit is pure aluminum oxide. This approach should be mechanically morerobust than the laminate. Other manifestations of this technique arepossible, including the use of different materials as well as alternatematerial variations.

The general fabrication process described in the previous patentapplications relies on the concept of surface micromachining, where asacrificial layer is deposited, a structure is formed on top of it, andthe sacrificial layer is etched away. One etch chemistry of particularinterest utilizes a gas-phase etchant to remove the sacrificial layer.Candidates include gases known as XeF2, BrF3, ClF3, BrF5, and IF5. Thesegases have the advantageous property of etching materials such assilicon and tungsten spontaneously, and without the need for a plasma toactivate the etch process. Because it is a gas phase etch, as opposed toa wet etch, the sacrificial etch step is much less complicated andprovides additional flexibility in the kinds of structural materialswhich may be used. Furthermore it facilitates the fabrication of moreelaborate devices with complex internal structures.

Display applications, in general, require the ability to fabricate onrelatively large substrates. While many finished display devices can besmaller than 1 square inch, most direct view displays start at severalsquare inches and can be as large as several hundred square inches orlarger. Additionally, these displays utilize glass or plastic substratesthat are not found in traditional semiconductor manufacturing plants.MEMS, which are primarily both silicon based and fabricated on siliconsubstrates, have been historically fabricated in semiconductor typefacilities. However the need to fabricate large arrays of MEM devices onlarge substrates, a need which is exemplified by an IMod based display,cannot be served using traditional semiconductor manufacturing practicesor facilities.

Alternatively, there exists a large and growing base of facilities thatcould also be applied to the manufacture of large arrays of IMods andother MEMS. This manufacturing base comprises facilities and factoriesthat are currently used to manufacture Active Matrix LCDs. The book“Liquid Crystal Flat Panel Displays”, by William C. O'Mara, isincorporated herein by reference. These facilities are appropriatebecause the bulk of the fabrication process is related to the activematrix component, i.e. the thin film transistor (TFT) array that drivesthe LCD.

While there exist a variety of TFT fabrication processes, they all shareseveral components which make them amenable to the fabrication of largearea surface micromachined MEMS. First, the substrate of choice is glassor plastic, which is readily available in large sized formats. Inaddition, key materials deposited include silicon, tungsten, molybdenum,and tantalum, all of which are suitable sacrificial materials for gasphase etchants, as well as tantalum pentoxide, silicon dioxide, siliconnitride, and aluminum, which are suitable optical, insulating,structural, optical, and conducting materials. In general, allphotolithography, process tooling, and testing are oriented towardslarge arrays and large area devices. Finally, the process forfabricating the TFTs can be utilized to fabricate electronics inconjunction with the MEM devices in order to provide driver circuitryand intelligent logic functions. Thus in conjunction with the gas phaseetch, Active Matrix LCD fabs and their associated processes provide areadily usable manufacturing vehicle for IMod based displays inparticular, and large area (at least as large of 14″×16″) MEM devices ingeneral.

Two general approaches for fabricating TFTs and IMods or other MEMdevices can be described as decoupled and overlapping. In the former therequisite TFT based circuitry is fabricated first, and then the IModsare fabricated subsequently. A more efficient approach is to fabricatethe TFT array and the IMod array in a way that allows the sharing oroverlapping of steps in each process. A representative TFT processsequence is shown in the following:

1. Deposit gate metal (molybdenum or tantalum for example).

2. Pattern gate metal.

3. Deposit insulator and amorphous silicon.

4. Pattern insulator and silicon.

5. Deposit display electrode (aluminum for example).

6. Pattern display electrode.

7. Deposit source/drain/signal line metal (aluminum).

8. Pattern source/drain/signal line.

9. Pattern silicon.

10. Deposit passivation film.

A representative IMod process sequence is shown in the following:

1. Deposit dielectric/primary mirror (molybdenum or tantalum for primarymirror).

2. Pattern primary mirror.

3. Deposit insulator and amorphous silicon.

4. Pattern insulator and silicon.

5. Deposit secondary mirror (aluminum)

6. Pattern secondary mirror.

7. Etch sacrificial material (silicon).

Comparison of these two process sequences reveals that steps 1-6 arefunctional equivalents on a fundamental level and, obviously, located atthe same place in their respective sequences. This similarity benefitsboth the decoupled and overlapping processes in several ways. First,similarity in materials minimizes the total number of dedicateddeposition tools required, as well as the number of etchant chemistries.Second, identical location of equivalent steps streamlines the overallprocess flow. Finally, for an overlapping process, some of the steps canbe shared. The consequence of this is an overall reduction in the totalnumber of process steps required to fabricate both the IMod array andthe TFT circuitry, reducing both complexity and cost. In general theprocess and facilities for manufacturing the active matrix component ofthe AMLCD would appear to be ideally suited for IMod fabrication.

Any thin film, medium, or substrate (which can be considered a thickfilm) can be defined in terms of a characteristic optical admittance. Byconsidering only the reflectance, the operation of a thin film can bestudied by treating it as an admittance transformer. That is, a thinkfilm or combination of thin films (the transformer) can alter thecharacteristic admittance of another thin film or substrate (thetransformed film) upon which it is deposited. In this fashion a normallyreflective film or substrate may have it's characteristic admittancealtered (i.e., transformed) in such a way that its reflectivity isenhanced and/or degraded by the deposition of, or contact with, atransformer. In general there is always reflection at the interfacebetween any combination of films, mediums, or substrates. The closer theadmittance of the two, the lower the reflectance at the interface, tothe point where the reflectance is zero when the admittances arematched.

Referring to FIG. 8, reflector 800 (the transformed film) is separatedfrom induced absorber 805 (the transformer), comprising films 804, 806,and 808, by variable thickness spacer 802. Incident medium 810 boundsthe other side of induced absorber 805. Each of these thin films ismicromachined in a fashion described in the parent patent application.Induced absorber 805 performs two functions. The first is to match theadmittances of reflector 800 and incident medium 810. This isaccomplished via matching layer 808, which is used to transform theadmittance of absorber 806 to that of the incident medium 810, and viamatching layer 804, which is used to transform the admittance ofreflector 800 to that of absorber 806. The second function is theabsorption of light. This is accomplished using absorber 806, whichperforms the function of attenuating light which is incident upon itthrough the medium, as well as light which is incident upon it from thereflector.

The ability to alter the thickness T of spacer 802 allows the opticalcharacteristics of the entire structure to be modified. Referring toFIG. 9, pixel 900 is shown in the driven state and pixel 902 in theundriven state. In this case induced absorber 906 (the transformer)resides on substrate 904 and reflector 908 (the transformed film) is aself-supporting structure. Application of a voltage causes reflector 908to come into contact or close proximity with induced absorber 906.Proper selection of materials and thickness will result in a completetransformation of the admittance of reflector 908 to that of substrate904. Consequently, a range of frequencies of light 905, which isincident through substrate 904, will be significantly absorbed by thepixel. With no voltage applied, reflector 908 returns to its normalstructural state which changes the relative admittances of the reflectorand the substrate. In this state (pixel 902) the cavity behaves morelike a resonant reflector, strongly reflecting certain frequencies whilestrongly absorbing others.

Proper selection of materials thus allows for the fabrication of pixelswhich can switch from reflecting any color (or combination of colors) toabsorbing (e.g., blue to black), or from reflecting any colorcombination to any other color (e.g., white to red). Referring to FIG.10, in a specific pixel design, substrate 1002 is glass, matching layer1004 is a film of zirconium dioxide which is 54.46 nm thick, absorber1006 is a tungsten film 14.49 nm thick, matching layer 1008 is a film ofsilicon dioxide 50 nm thick, spacer 1000 is air, and reflector 1010 is afilm of silver at least 50 nm thick. Referring to FIG. 10, the opticalresponse of the pixel is shown in the driven state, i.e., when reflector1010 is in contact with matching layer 1008 resulting in a broad stateof induced absorption. Referring to FIGS. 12-15, the different colorpixels are shown in respective undriven states which correspond to thereflection of blue, green, red, and white light, respectively. Theseresponses correspond to undriven spacer thicknesses of 325, 435, 230,and 700 nm, respectively.

Referring to FIG. 16, a section of full color reflective flat paneldisplay 1600 includes three kinds of pixels, R, G, and B. Each kinddiffers from the others only in the size of the undriven spacer which isdetermined during manufacture as described in the parent patentapplication. Induced absorber 1602 resides on substrate 1606, andreflector 1610 is self-supporting. Monolithic backplate 1604 provides ahermitic seal and can consist a thick organic or inorganic film.Alternatively, the backplate may consist of a separate piece, such asglass, which has been aligned and bonded to the substrate. Electrodesmay reside on this backplate so that the electromechanical performanceof the pixels may be modified. Incident light 1612 is transmittedthrough optical compensation mechanism 1608 and substrate 1606 where itis selectively reflected or absorbed by a pixel. The display may becontrolled and driven by circuitry of the kind described in the parentpatent application.

Optical compensation mechanism 1608 serves two functions in thisdisplay. The first is that of mitigating or eliminating the shift inreflected color with respect to the angle of incidence. This is acharacteristic of all interference films and can be compensated for byusing films with specifically tailored refractive indices or holographicproperties, as well as films containing micro-optics; other ways mayalso be possible. The second function is to supply a supplementalfrontlighting source. In this way, additional light can be added to thefront of the display when ambient lighting conditions have significantlydiminished thus allowing the display to perform in conditions rangingfrom intense brightness to total darkness. Such a frontlight could befabricated using patterned organic emitters or edge lighting sourcecoupled to a micro-optic array within the optical compensation film;other ways may also be possible.

The general process for fabrication of the devices is set forth in theparent patent application. Additional details of two alternative ways tofabricate spacers with different sizes are as follows; other ways mayalso be possible.

Both alternative processes involve the iterative deposition andpatterning of a sacrificial spacer material which, in the final step ofthe larger process is, etched away to form an air-gap.

Referring to FIG. 17A, substrate 1700 is shown with induced absorber1702 already deposited and photoresist 1704 deposited and patterned.Induced absorber 1702 is deposited using any number of techniques forthink film deposition including sputtering and e-beam deposition. Thephotoresist is deposited via spinning, and patterned by overexposure toproduce a natural overhang resulting in a stencil. The result is that itmay be used to pattern subsequently deposited materials using aprocedure known as lift-off. Referring to FIG. 17B, spacer material 1706has been deposited, resulting in excess spacer material 1708 on top ofthe stencil. Referring to FIG. 17C, the stencil along with the excessspacer material have been lifted off by immersing the device in a bathof solvent such as acetone and agitating it with ultrasound. Referringto FIG. 17D, the process has begun again with new photoresist 1710having been deposited patterned in a fashion such that new spacer 1712is deposited adjacent to the old spacer 1706. Repeating the process oncemore results in spacers with three different thicknesses. Referring toFIG. 17D, the process has begun again with new photoresist 1710 havingbeen deposited patterned in a fashion such that new spacer 1712, with adifferent thickness, is deposited adjacent to the old spacer 1706.

Referring to FIG. 18A, substrate 1800 is shown with induced absorber1802 already deposited. Spacer materials 1804, 1806, and 1808 have alsobeen deposited and patterned by virtue of lift-off stencil 1810. Thespacer materials have a thickness corresponding to the maximum of thethree thicknesses required for the pixels. Referring to FIG. 18B, thestencil along with the excess material has been lifted off and newphotoresist 1812 has been deposited and patterned such that spacer 1804has been left exposed. Referring to FIG. 18C, spacer material 1804 hasbeen etched back via one of a number of techniques which include wetchemical etching, and reactive ion etching. Only a portion of therequired spacer material is etched away, with the remainder to be etchedin a subsequent etch step. Photoresist 1812 is subsequently removedusing a similar technique. Referring to FIG. 18D, new photoresist 1814has been deposited and patterned exposing spacers 1804 and 1806. Theentire etch of spacer 1806 is performed in this step, and the etch ofspacer 1804 is completed. Photoresist 1814 is subsequently removed andthe process is complete.

For example, the spacer material need not ultimately be etched away butmay remain instead a part of the finished device. In this fashion, andusing the previously described patterning techniques, arbitrary patternsmay be fabricated instead of arrays of simple pixels. Full color staticgraphical images may thus be rendered in a method which is analogous toa conventional printing process. In conventional printing, an image isbroken up into color separations which are basically monochromegraphical subsets of the image, which correspond to the different colorsrepresented, i.e., a red separation, a blue separation, a greenseparation, and a black separation. The full-color image is produced byprinting each separation using a different colored ink on the same area.

Alternatively, in a process which we will call “Iridescent Printing”,the different separations are composed of layers of thin films whichcorrespond to the IMod design described here and those in the referencedpatent. Patterning or printing a combination of colors or separations onthe same area, allows for brilliant fill-color images to be produced.

Referring to FIG. 19A, a square substrate is shown with area 1900representing the portion of the substrate which has been patterned witha thin film stack optimized for black. Referring to FIG. 19B, thesubstrate has been subsequently patterned with a thin film stackoptimized for red in area 1902. Referring to FIG. 19C, the substrate hasbeen subsequently patterned with a thin film stack optimized for greenin area 1904. Referring to FIG. 19D, the substrate has been subsequentlypatterned with a think film stack optimized for blue in area 1906.

Alternatively, a simpler process can be obtained if only the inducedabsorber design is used. In this process, the entire substrate is firstcoated with the induced absorber stack. Subsequent steps are then usedto pattern the spacer material only, using the aforementionedtechniques. After the desired spacers, i.e., colors are defined, a finaldeposition of a reflector is performed.

The brightness of different colors can be altered by varying the amountof black interspersed with the particular color, i.e., spatialdithering. The images also exhibit the pleasing shift of color withrespect to viewing angle known as iridescence.

In another example, a reflective flat panel display may also befabricated using a single kind of pixel instead of three. Multiplecolors, in this case, are obtained through fabricating the pixels in theform of continuously tunable or analog interferometric modulators asdescribed in the parent patent application. In this fashion, anyindividual pixel may, by the application of the appropriate voltage, betuned to reflect any specific color. This would require that the arraybe fabricated on a substrate along with electronic circuitry, ordirectly on the surface of an integrated circuit, in order to provide acharge storage mechanism. This approach, though it requires a morecomplicated driving scheme relying on analog voltages, provides superiorresolution. It would also find application in a projection system.

Other embodiments are within the scope of the following claims.

1. A method of fabricating a microelectromechanical structure (MEMS),the method comprising etching a deposited sacrificial layer with anon-plasma gas phase etchant, wherein the sacrificial layer comprises atleast one of molybdenum, tungsten, or tantalum, wherein the MEMScomprises an interferometric modulator, further comprising forming awall of the interferometric modulator on the deposited sacrificial layerprior to etching.
 2. The method of claim 1, wherein the non-plasma gasphase etchant comprises at least one of XeF₂, BrF₃, ClF₃, BrF₅, and IF₅.3. The method of claim 1, further comprising chemically isolating thesubstrate from the MEMS.
 4. A method of forming a movablemicroelectromechanical device comprising: disposing a sacrificial layeron a substrate, wherein the sacrificial layer comprises at least one ofmolybdenum, tungsten, or tantalum; disposing a microelectromechanicalstructure over the sacrificial layer and the substrate, themicroelectromechanical structure comprising at least one thin film; andetching the sacrificial layer with a non-plasma gas phase etchant torelease the microelectromechanical structure from the sacrificial layer,wherein the microelectromechanical device comprises an interferometricmodulator and the microelectromechanical structure comprises a wall ofthe interferometric modulator.
 5. The method of claim 4, wherein thenon-plasma gas phase etchant comprises at least one of XeF₂, BrF₃, ClF₃,BrF₅, and IF₅.
 6. The method of claim 4, further comprising chemicallyisolating the substrate from the microelectromechanical device.
 7. Themethod of claim 6, wherein chemically isolating comprises depositing athin film coating on the substrate.
 8. The method of claim 1, whereinthe sacrificial layer comprises molybdenum.
 9. The method of claim 1,wherein the sacrificial layer comprises tungsten.
 10. The method ofclaim 1, wherein the sacrificial layer comprises tantalum.
 11. Themethod of claim 4, wherein the sacrificial layer comprises molybdenum.12. The method of claim 4, wherein the sacrificial layer comprisestungsten.
 13. The method of claim 4, wherein the sacrificial layercomprises tantalum.
 14. The method of claim 1, wherein the non-plasmagas phase etchant comprises XeF₂.
 15. The method of claim 4, wherein thenon-plasma gas phase etchant comprises XeF₂.
 16. The method of claim 1,wherein the wall of the interferometric modulator is at least partiallyreflective and at least partially transmissive.
 17. The method of claim4, wherein the wall of the interferometric modulator is at leastpartially reflective and at least partially transmissive.